The present invention is directed to a method for rehabilitating internally reinforced concrete by removal of chlorides.
One of the serious problems in connection with the maintenance of reinforced concrete is the matter of corrosion of the internal reinforcement. In many cases corrosion of the reinforcement is caused by the chloride contamination. This may result from the gradual absorption of chlorides over a period of time or, in some cases, from the incorporation of chlorides into the original concrete to accelerate setting. Conventional techniques for repairing chloride-contaminated concrete involve physical removal of the contaminated material and replacement with fresh concrete. This is obviously a costly and disruptive solution, at least with respect to vertical and overhead surfaces.
It has been proposed heretofore to employ electrolytic techniques for removing chlorides by ion migration. An article by J. E. Slater, Materials Performance, 1976, pp 21-26, describes such a method, which involves applying an electric potential between internal reinforcement and a surface electrode submerged within a liquid electrolyte contained on the surface of the concrete. With the surface electrode forming the positive pole of the electric field, the negative chloride ions within the concrete are caused to migrate through the concrete and out into the electrolyte where they are oxidized to chlorine gas on the positive electrode or react chemically with components in the electrolyte.
The experiments described in the Slater article were carried out on chloride contaminated bridge decks, where the reinforcement had corroded. The bridge deck area was divided into sections of about 3.5 m.sup.2 that were individually treated. These sections were provided with sealed dams for containing a liquid electrolyte solution. The electrolyte used was a calcium hydroxide solution, with and without ion exchangers. Slater used voltages of between 100 and 120 V, and the current varied between 28 and 100 amps per section. Slater was able to remove up to 90% of the chlorides in the concrete within a 24 hour period. Where the electrolyte was used without ion exchangers, chlorine gas developed on the platinized titanium electrodes and was released as free chlorine gas.
The Slater technique has not been commercially successful, for several reasons: For one, the safety considerations are substantial when operating at voltages as suggested by Slater. More importantly, the Slater procedure is useful only for removing chlorides from the upper surface of a horizontal slab. However, the more conventional method of removing the concrete is relatively simple and inexpensive when dealing with an upwardly facing horizontal slab. The Slater method may well be more costly than conventional techniques.
The present invention provides an economical electrolytic method of removing chlorides from contaminated reinforced concrete that can be carried out safely and with reasonable energy requirements and which, importantly, can be utilized conveniently and economically on vertical and downwardly facing surfaces. In U.S. Pat. Nos. 4,832,803 and 5,198,082 (Ser. No. 366,204), mentioned above, it is proposed, generally, to employ removable, cohesive, self-adherent materials as electrolytic media in electrolytic chloride removal procedures. A distributed electrode means is embedded within the adherent coating and forms the positive terminal of the electrical system. When the process has been completed, that is, when the level of chloride contamination has been reduced to a desired level, both the adherent coating and the electrode means are removed from the face of the concrete. In this respect the process of the invention differs markedly from cathodic protection systems, for example, where systems are permanently installed for continuously maintaining an electric potential between internal reinforcement and surface electrode means.
In the above-mentioned U.S. Pat. No. 4,832,803, a material such as retarded gunite is disclosed as one of the materials suitable for use as an adherent electrolyte coating having adequate conductivity and being removable upon completion of the procedure. In an especially advantageous and still further improved procedure according to the present invention, the adherent electrolytic coating material is formed of a mixture of cellulosic pulp fiber and water or other solution, which is self-adherent to the surface of the concrete. The pulp fiber, which is advantageously derived from re-cycled newsprint, is mixed with the liquid solution at the outlet of a spray nozzle, and the fiber-liquid pre-mix is sprayed onto the surface of area to be treated. The surface of the concrete draws some of the solution from the sprayed mixture and causes the mixture to adhere tenaciously to the concrete surface.
Pursuant to the invention, the concrete area to be treated is tested by the taking of cores and testing for chloride content. From these initial tests, the approximate time required to achieve a desired level of chloride reduction can be estimated. The treatment can be continued until approximately the estimated time has passed, after which a further set of cores may be taken to establish the final processing conditions.
For a more complete understanding of the features and advantages of the invention, reference should be made to the following description of preferred embodiments and to the accompanying drawing.